This Battlefield 3 video Q&A features GameStar's interview with EA/DICE's Patrick Söderlund talking about Battlefield 3 (thanks GameSpy). One of the community questions they ask is about whether they plan to release mod tools with the game, to which Patrick says no: "Well as of now we're not going to make any modding tools, no." Here's his reply when asked why: "'Cause if you look at the Frostbite engine, and how complex it is, it's going to be very difficult for people to mod the game. Because of the nature of the set up of levels, the destruction, and all those things, it's quite tricky, so we think it's going to be too big of a challenge for people to make a mod."

As others have mentioned, you don't necessarily need mod tools in order to mod a game. Mod tools just make it a lot easier. Dead Rising 2 had mods. RE4 had mods. Tomb Raider: Underworld had mods. STALKER had mods. Fallout 1&2 had mods. Vampire: The Masquerade - Bloodlines had mods. GTA4 had mods. Carmageddon had mods.